Wide band amplifier



Aug. 18, 1942.

w. R. KOCH WIDE BAND AMPLIFIER Fiid' m 29, 1940 I arr/005451 0 waver/swas- Gttomeg Patented Aug, 18,

UNITED STATES PATENT orr ce WIDE BAND AMPLIFIER Winfield a. Koch, Haddonfield, N. 1., assignor to Ameriea,a corporation oi Radio Corporation oi Delaware Application February 29,1940, Serial No. 321,543

5 Claims. Cl. 179171) v The present invention relates to .wide band. high frequency amplifiers, and has for an object The amplifier stage includes a suitable amplifier tube 8 having a signal input grid 9 connected with to provide an improved wide band amplifier and tential of an amplifier for volume control purposes tend to change the input loading caused by the grid of the tube, the loading'being greater the higher the frequency at which the amplifier operates, and increasing as the bias potential is decreased.

It has been found that the loading of a signal conveying circuit such as a signal input grid circuit is caused partly by the electron transit time and partly by the inductance of the cathode lead at high and ultra high frequencies, and it is a further object of the present invention, to provide an improved amplifier having a tuning network in one or more of the stages thereof for compensating the cathode lead inductance in such -a manner that the loading of the input circuit may be made substantiallyuniform overa relatively wide frequency band and may'provide additional compensation for the transit time of the electrons, i

By the use of the present invention a multistage ultra high frequency amplifier may be provided with volume control involving grid bias variation, from AVC means, for example, without changing the effective loading of the input circuits" of the various stages thereof when operating over a relatively wide band of signalfren quencies. i The invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope is pointed out in the appended claims.

In the drawing, Figure 1 is a schematic circuit diagram of a high frequency wide band amplifier embodying the invention; and

a high impedance tuned input circuit ill formed by the secondary ll of the transformer 6 and a' shunt capacity provided by a capacitor I 2, A limiting resistor i3 is provided across the tuned input circuit which is connected at its low potential side to a variable source of biasing potential I! through a filter resistor IS.

The cathode inductance or the inductance of the cathode lead is indicated as a unit element 16 connected betweenthe cathode l1 and a connection with ground l8 through a filter resistor l9.

In the present example, the biasing potential as an impedance between the cathode and ground, v

the cathode circuit maybe made efiectively resistive in the pass band of the amplifier unifor'mly throughout the pass band. Such a network is shown in the present example and comprises the bias supply to the grid 9 is suitably filtered by the resistor IS in conjunction with a by-pass capacitor 22. Ihe filter resistor I9 is effectively the output resistor of the low pass filter structure.

In a wide band amplifier, for example, one amplifying uniform signal frequencies between 19 and 31 megacycles, the loadingof the input circuit l0 because of the grid 9 of the amplifier tube 8, is considerably greater for the upper-endof the hand than for the lower end thereof. At

normal bias, this may be compensated by slight poses, as by adjustment of the contact the equivalent shunt resistance offered by the grid of.the tube will increase as the bias on'the tube is increased and the change in loading will be greater at the high frequency end of the band and the selectivity characteristic will tend to rise at the high frequency end of the band. This is for the reason that inductance in the cathode lead or in the "cathode has the effect on the grid cirsame as in Fig. 1. further detailed I unnecessary cult of a shunt resistance whichvaries with frequency and bias. In the present example, the upper limit of this resistance is determined by the resistor II which may be set, for example, at 500 ohms.

The reactance oi the cathode circuit increases with frequency and thus increases the effective shunt resistance across the grid circuit, the value varying inversely as the square of the frequency. This may be of the order of 20,000-ohms at the upper limit of the band hereinbefore referred'to,

such as 31 megacycles, and may be 100,000 ohms at the lower end of said band, such as 19 megacycles.

In accordance with the invention, the efiective loading of the input circuit is maintained substantially uniform with changes in bias control by means of the low pass filter structure or network provided in the cathode circuit which has, in the present example, a cut-off above 31 mega- .cycles, whereby the cathode circuit is made effectively resistive throughout the band. Whilethe :variation in biasing potential under such conditionstends to cause some changes in the loading the minimum is relatively high, such as 100,000 ohms. Therefore, ithas little eii'ect upon the circuit which has a relatively low resistance of a few hundred ohms in a wide band amplifier.

Referring to Fig. 2, wherein like reference numerals are applied to the same circuits and elements as in Fig. 1, the cathode circuit is modified to include in series therewith, a shunt 2. In a wide band high frequency s nal amplifier, the combination with an amplifier tube having a signal input grid, a cathode and a cathode lead, of means providing a signal input circuit connected between said grid and cathode through said lead, means for applying a variable biasing potential between said grid and cathode, thereby to vary the gain of said amplifier, and means for preventing the cathode lead in ductance of the said tube from affecting the uniform signal amplification of said amplifier over the frequency band thereof, comprising a low pass filter network including the cathode lead inductance, connected as an impedance in circuit between said cathode and ground.

3. ma wide band signal amplifier for high fre-- in said circuit, said filter network having a cutoff frequency above the pass band of the ,amplifier.

4.'In a wide band, high equency amplifier comprising a plurality of amplifier stages, the combination with at least one of said stages of -means providing a tuned input ,grid circuit,

tuned inductance II and capacity 20, bypassed to ground by a capacitor 24 together with a variable biasing potential source comprising bleeder resistors 21 and II. The resistor II is variable to control the fiow of current from a positive terminal I! to ground ll, thereby to de- -ve'lop a biasing potential across the resistor section 21 for the grid. The bias potential is supplemented by a variable AVC potential applied to the grid from a, supp y lead ll across a grid resistor 32, the .latter being connected in series between the low potential side of the tuned input circuit II and mud ll. Since the operation and Purpose of this circuit is otherwise the description is variable biasing potenti l on the con trol grid I, from either the variable source 21 or theAVC source 8|, the band pass characteristic of the amplifier and the uniform amplification of signals therein is substantially unchanged,

' so that eifective AVG control maybe obtained without undesired loading eifects. j

Iclaimasmyinventio'm" i. In a wide band. signal amplifier for high frequency signals, the combination of an amplifier tube having a cathode and a cathode lead, a signal input circuit therefor. a signal output circuit therefor, and means for compensating the cathode iead inductance of said tube comprising a low pass filter network in circuit with the cathode and including the cathode lead inductance.

,means for applying a variable biasing potential to said circuit for effecting volume ,control of said amplifier stage, a cathode circuit for said stage having a predetermined inherent inductance in operation efi'ective to cause a variation of the input grid circuit loading with variations in fre-' quency and in biasing potentiahand means for minimizing the efifect of saidHnduetance on the 1 loading of said circuit comprising a low pass filter'network including said inductance and a filter resistor as the output element thereof in said cathode circuit, said filter network having a cut-off frequency above the operating frequency band of the amplifier.

5. In a high frequency wide bandamplifier, the combination of an amplifier stage comprising a tube having a,cath ode,-and a cathode lead which is effective asan inductance at the operating frequency of the amplifier, means for applying a varying biasing potential to said stage for controlling the volume level of amplified signals, and means for maintaining the effective shunt loading of said input circuit relatively high with respect to the effective resistance of said circuit with variations in biasing potential, comprising a low pass filter network in circuit with the cathode and including the cathode lead inductance, said network having a cut-off frequency above the pass band of the amplifier, thereby to cause saidcathode circuit to be efiectively resistive in said pass band of the amplifier. 

